Field of the Invention
The present invention relates to a hydrogen concentration measuring device adapted to measure a hydrogen concentration by using a hydrogen sensor capable of detecting hydrogen.
Description of the Related Art
Hitherto, as one type of hydrogen sensor used with a hydrogen concentration measuring device, there has been known a contact combustion type. The contact combustion type makes use of the combustion heat generated from the contact combustion of a hydrogen gas, and has a high detection sensitivity. The contact combustion type is used mainly to measure a hydrogen gas in a low concentration range of a few thousand ppm to a few ten thousand ppm.
Further, there is a method for measuring the concentration of a hydrogen gas called “the gas thermal conductivity method.” This method makes use of the difference in thermal conductivity between a gas to be measured and a reference gas (e.g. atmospheric air). This method enables the measurement in an extensive concentration range from 1% to 100%, although the detection sensitivity is low.
Currently, an optical type is most typical for measuring the concentration of a hydrogen gas. For example, there has been known an optical hydrogen sensor that uses tungsten trioxide WO3 or vanadium pentoxide V2O5 as a hydrogen gas sensing film.
The optical hydrogen sensor utilizes the fact that the exposure of the sensing film to a hydrogen gas changes the optical characteristics of the sensing film, thereby enabling the hydrogen gas to be detected from the attenuation of transmitted light or reflected light (optical loss). In particular, one or two layers of an alloy catalyst film (e.g. a combination of palladium Pd and rhodium Rh) are placed on the sensing film to enhance the responsiveness of the sensing film (refer to, for example, Japanese Patent Application Laid-Open No. 2007-57233).
However, the optical hydrogen sensor is known to have a relatively low detection sensitivity in a low concentration range while having a high detection sensitivity in a high concentration range. Although it is possible to perform the measurement itself in a low concentration range by the optical hydrogen sensor, there has been a problem in that the measurement in a low concentration range takes a longer time than the measurement in a high concentration range.
It could be possible to construct a hydrogen concentration measuring device capable of performing measurement in an extensive concentration range by combining different types of hydrogen sensors. However, combining different types would inconveniently make the device configuration complicated. In addition, there has been a problem in that, if there are concentration measurement values obtained by different types, then a difference in measured concentration will inconveniently occur at the time of changing the type to use.